1. Field of the Invention
This invention pertains in general to encoding digitized images and in particular to a method for encoding different regions of an image at different levels of image quality.
2. Description of Background Art
The medical field often relies on images, like those from X-ray photography and magnetic resonance imaging (MRI), to assist in the diagnosis and treatment of patients. These images are often acquired and stored digitally. For example, modem X-ray systems capture the X-rays on a digital plate and produce a digital image therefrom. Even images from analog sources are frequently digitized in order to facilitate storage and retrieval.
These digital images are often quite large. An uncompressed grayscale image having a resolution of 3000.times.2000.times.12-bit pixels requires about nine megabytes of storage space and a typical image storage system may store thousands of such images. In response, image compression techniques have been developed to reduce the amount of space needed to store digital images. One well-known compression technique is Joint Photographic Experts Group (JPEG) encoding.
JPEG encoding is either "lossy" or "lossless." In lossy encoding, some high-frequency image information is discarded. Lossy compression can be used to obtain high compression rates with only a negligible decrease in decoded image quality. In lossless encoding, all image information is preserved and the decoded image quality is unchanged from that of the source. Lossless encoding, however, does not compress the source image to the same extent as lossy encoding.
Certain fields, like the medical field, resist using lossy encoding due to a concern that important image information will be lost. Accordingly, even lossy encoding in which the amount of lost information is too small to make a practical difference is discouraged.
Nevertheless, many medical images can be roughly divided into important regions containing information relevant to diagnosis or treatment and less important regions containing information of lessor relevance. For example, in an X-ray of a broken bone, the region of the image showing the break is an important region while the surrounding regions, assuming the regions do not show anything remarkable, are less important regions. Although less important regions should be retained in the image in order to provide context to the important regions, the less important regions need not be retained with the same fidelity as the important regions. In addition, these distinctions are also applicable to other fields wherein a large number of digitized images must be stored yet the image can be divided into important and less important regions.
JPEG encoding, however, does not specify a technique for encoding different regions of an image at different levels of image quality. Although the JPEG standard includes a "hierarchical mode" wherein an image is encoded into a reference frame and differential frames successively refining the reference frame, the hierarchical mode operates on the entire image and does not contemplate encoding different regions of an image at different image quality levels. Hierarchical coding is discussed in detail in William B. Pennebaker and Joan L. Mitchell, JPEG Still Image Data Compression Standard, Van Nostrand Reinhold, 1993, which is hereby incorporated by reference herein. Therefore, there is a need for an image encoding technique that allows different regions of an image to be encoded at different levels of image quality.